what is the difference in these two isinstances() definitions?

Question

isinstance(list(), type(mylist))
isinstance(mylist, list)

Is there any difference apart from the speed? I can see 2 is faster.

how did you define `array`? or do you mean the `array.array`? — Chris_Rands, Nov 19 '18 at 15:09
The first one is making a new list and checking if is an instance of the type of `array` (whatever that is). The second is checking if `array` is an instance of list. They are not equivalent. — khelwood, Nov 19 '18 at 15:12
so you have created a variable called `my_list` which is a Python `list` and you are trying to figure out what the fastest way to check its type is. Correct? — Ma0, Nov 19 '18 at 15:12
2. is better, don't add unnecessary function/class calls for no reason — Chris_Rands, Nov 19 '18 at 15:13
@Ev.Kounis I measured that 2 is faster. I am actually wondering what the difference is apart from speed — user2715898, Nov 19 '18 at 15:13
@user2715898, The difference is you can *easily* use the first one incorrectly, see my example. It's not *just* about speed. — jpp, Nov 19 '18 at 15:20

score 3 · Accepted Answer · answered Nov 19 '18 at 15:17

3

If used correctly, the results will be the same. But you should only use the second version.

There's no need to create an empty object, i.e. list(), and no reason for an extra function call via type.

There's also a danger you'll do something like this:

from collections import OrderedDict

array = dict()

res1 = isinstance(OrderedDict(), type(array))  # True
res2 = isinstance(array, OrderedDict)          # False

answered Nov 19 '18 at 15:17

jpp

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This is a good point, the results are not always symmetric because of subclasses, of course we don't what they actually want in this case – Chris_Rands Nov 19 '18 at 15:20
@Chris_Rands, Yup, it's just an illustration of *what's more likely* to go wrong with the first method. – jpp Nov 19 '18 at 15:21

score 1 · Answer 2 · answered Nov 19 '18 at 15:15

1

The first one is very inefficient, as you are first creating a new list object, and then identifying the type of my_list and then checking if they both are same.

The second option is how the isinstance function is supposed to be used.

answered Nov 19 '18 at 15:15

Aswin Murugesh

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what is the difference in these two isinstances() definitions?

2 Answers2